PURPOSE: To passivate a glass optical waveguide and to reduce hydrogen-induced loss at the time of using in a hydrogen-containing atmosphere by heat-treating the glass optical waveguide in advance in a deuterium-containing atmosphere at a specific temp. before the glass optical waveguide is used in the hydrogen-containing atmosphere.

CONSTITUTION: An optical fiber preform having a central core region and an outer clad is manufactured, and line-drawn to prepare an optical fiber, and it is coated with a protective polymer to form the glass optical waveguide. Then, the optical waveguide is arranged in the deuterium-containing atmosphere containing deuterium of partial pressure of ≤10 atmospheric pressure, is heated at a temp. within the range of 50 to 200°C to passivate the optical waveguide. Then, obtained optical waveguide is used in the hydrogen-containing atmosphere. Thereby the hydrogen-induced loss at the time of using in the hydrogen-containing atmosphere can be reduced owing to preliminary heat-treatment in the deuterium-containing atmosphere. Further, the heat-treatment is suitably executed in the deuterium-containing atmosphere of atmospheric pressure for ≥12 hours or in an atmosphere containing deuterium of partial pressure of over 1 atmospheric pressure.